Advances in composite technology have had a marked impact on product design and engineering and, ultimately, manufacture. Early methods involving hand lay-up of fibrous materials and sheets with the subsequent impregnation of the laid up materials and, subsequently, the laying up of pre-impregnated fibrous sheets and mats followed by compression forming and curing saw rapid adoption and exploitation of these composite materials and technologies in many fields. However, while useful for many applications, the slow methodical build-up of the layers of materials is very labor intensive, oftentimes involves the use of hazardous chemicals and, more importantly, employs unstable materials and/or materials having limited working time. Thus, while a marked advance in the industry, their applications were still limited and costly.
Subsequent advances in composite materials and technology led to continuous manufacturing techniques. Perhaps the most notable of these techniques is filament winding wherein a continuous tow of certain fibrous materials were pulled though a bath of a curable material to impregnate the same with the curable material, the wet tow is then wound about a mandrel to form the desired part, preferably with some measure of immediate cure (oftentimes a UV cure) to attain a green state so as to prevent excessive flow of the resin material and maintain the fiber placement during the winding process following which the wound structure is fully cured. However, these operations are very slow and time consuming owing, in large part, to the narrow width of the tow. More importantly, these processes are very capital intensive as the whole of the operation, from preparation of the curable composition to the wetting of the tow of fiber material and, subsequently, to the formation of the part itself, all had to be performed in the same immediate vicinity of one another, typically on the same floor space.
Continued advances in both manufacturing and materials technology led to the ability to prepare cut sheets and, subsequently, master rolls of “green state” or partially cured prepreg materials: the latter in the form of continuous strips and sheets of tows of woven fibers or unidirectional, parallel fibers, most typically glass or carbon fibers, impregnated with a thermoset/thermosetting resin, especially epoxy, BMI or Polyimide resins. These prepreg materials are generally formed on a support sheet material, typically paper, which also serves as a separator or liner material to prevent successive layers, in the case of stacked cut sheets, or successive windings, in the case of master rolls, of the prepreg materials from contacting and adhering or melding to one another. The so formed cut sheets and master rolls of prepreg materials are then stored and shipped to the ultimate end-user for use in manufacturing composite structures. Owing to the unstable nature of the thermoset/thermosetting resin material, these cut sheets and master rolls are stored and maintained at significantly reduced temperatures, oftentimes at or below freezing, until use or further processing so as to prevent or at least significantly retard premature curing of the thermoset resin. The inherent cost advantages and focus on centered expertise (e.g., prepreg manufacturers concentrated on the chemistry and manufacture of the prepreg materials and the product manufacturers concentrated on the layup process) proved beneficial all around. No longer was it necessary for the ultimate composite product manufacturer to invest in the capital equipment, consumable materials, overhead, spatial requirements, and technical expertise and personnel to make the prepreg materials.
Advances have also been made whereby the prepreg materials may be formed using a thermoplastic resin, rather than a thermoset/thermosetting resin. Here the thermoplastic polymer is heated to a soften, flowable or at least readily pliable state and combined with and infiltrated through the fibers. This may be accomplished by many known methods including extrusion/co-extrusion, pultrusion, vacuum/pressure molding, injection molding, etc., to form the prepreg materials, also known as composite materials. These prepreg materials are mated with a liner as they are being formed or subsequent thereto but prior to stacking or rolling.
Early on, master rolls of prepreg strip or sheet materials were manufactured in standard widths that were then used to make the commercial products. This was acceptable as many applications had very similar demands and requirements, e.g., baseball bats, golf clubs, hockey sticks, lacrosse sticks and the like can all be made with generally the same width of slit tape, largely because the demands are similar and the need for differentiation less. Even if not optimal, these stock rolls were used nonetheless as making many different widths, especially making custom widths, was cost prohibitive: the capital requirements are quite large and incapable of supporting a large differentiation of product widths. Consequently, the full adoption of this technology in higher demanding and higher tech applications was limited if the width of the tapes needed were inconsistent with the widths that were available.
More recently, the technology has evolved and new expertise and processing capabilities have been developed whereby a broad array of tape widths are made possible through the slitting of stock master rolls. Specifically, slitting apparatus and equipment has been developed which allows one to unwind the master rolls of prepreg material, remove the liner, pass the prepreg material through a slitter whose cutting elements, e.g., knives or blades, are spaced to produce the desired widths of tape, and rewind the so formed prepreg slit tape, commonly referred to as just “slit tape,” with a new liner, preferably a polymer liner. This innovation allowed for the production of wide slit tape for use in the manufacture of large planar or curved planar surfaces as well as narrow width slit tape for use in the manufacture of more intricate parts and/or parts having multiple changes in surface orientation.
Although the advent of slit tape allowed for the use of prepreg materials in the manufacture of many different products whose demands, especially physical demands, required specific properties which are affected by, in part, the width of the prepreg slit tape, limitations still existed. Specifically, the extension of the use of slit tape into high tech applications, especially in the production of components for aerospace and aircraft production, has placed increased demands on consistency and tight tolerances in the slit tape, both in terms of the make-up and dimensions of the slit tapes. While one might think that the demands for aerospace and aircraft production would be fairly constant, nothing could be further from reality. Each component for aerospace and aircraft production must endure a combination of environment conditions and physical demands and stresses that are most often unique to that component. For example the requirements of the tail fin of an aircraft will be markedly different from those of a wing or a fuselage portion. Similarly, the shapes of such components vary from generally smooth, curved structures, as with a fuselage, to multi-oriented surfaces with sharp angles, as with a tail fin.
In order to meet the requirements for aircraft and aerospace applications, it has become more and more apparent that a plethora of widths of slit tape are needed with differences in the width of the slit tape from one application to the another oftentimes being on the order of just fractions of an inch. Furthermore, and perhaps more importantly, these applications demand extremely tight tolerances in relation to any variation in the width of the slit tape for a given application, with tolerances being orders of magnitude smaller than for most commercial applications. In aerospace, it is not uncommon for tolerances in width to be on the order of thousandths, if not ten-thousandths, of an inch.
However, the requirements do not end there as slit tape for these and other high tech applications must also be free of artifacts, whether of foreign origin or which original from the master prepreg materials themselves. The former can be controlled by proper environmental controls and clean-room type practices. The latter, however, requires care in the selection and implementation of materials, apparatus and methods or processes. For example, poor quality cutting elements, e.g., knives or blades, and/or improper or too infrequent cleaning thereof may lead to a buildup of resin on the blade which, in turn, may attach to or fall on the slit tape as it moves out of the slitter. While such artifacts are generally not problematic in the winding of the slit tape, the concern here is that such artifacts may create or lead to a potential failure point in the part to the made.
As noted above, slit tape is typically formed by unwinding the master roll of prepreg material, removing the backing (which also acts as a liner for the master roll), passing the prepreg material through a slitter whose cutting elements, e.g., knives are spaced to produce the desired widths of slit tape, and rewinding the so formed slit tape with a new liner. Removing the backing from the master roll prior to the slitting operation is critical in order to meet the tight tolerances and artifact free requirement for high tech applications, especially aerospace and aircraft production.
The most common backing material used in the production of master rolls is paper. Because paper is comprised of fibrous materials whose orientation is random, it is difficult, if not impossible, to avoid the formation or accumulation of fibrils along the cut edge of the paper as well as the generation of free fibrils and paper dust which become airborne. These loose fibrils and the paper dust then adhere to and comprise undesired artifacts on the slit tape itself. The degree to which such fibrils and paper dust form is, in part, dependent upon the quality and condition of the slitting knives or blades themselves, with more being formed as the knives or blades become worn. Of course, it is to be appreciated that the coarse nature of the paper would also hasten the wearing of the knife of blade.
In addition to an increase in the generation of fibrils and paper dust, poor quality and/or worn knives and blades leads to or increases the rate of the buildup of fibrils and/or resin on the blade, which affects the consistency of the cut as well as, again, the deposit of artifacts on the slit tape. Furthermore, when paper is moving across a stationary knife or blade, especially one that is worn, there is the tendency for the paper to catch on the knife or blade. This happenstance may lead to a tear in the paper and/or, more critically, an unexpected jolt, snag, or stall in the slitting process, which, in turn, causes a movement in the prepreg and slit tape as it is being formed and, hence, a variation in the width of the slit tape and/or a tear or break in the slit tape itself.
Although paper as the backing still dominates, one or two manufacturers have recently experimented with and produced prepregs using a polymer backing, a polymer film or fabric, owing to their cost-effectiveness. However, like paper, there are no set requirements as to the integrity, nature, orientation, or physical properties of these polymer backings or their peel strength relative to the prepreg material as their sole purpose is merely to serve as the foundation in the manufacture of the prepreg material and, subsequently, the separator between successive windings thereof in forming the master rolls. However, while cost effective, polymer backings have some similar problems as well as unique problems associated with their use. For example, unlike paper, which lies flat and has a consistent tautness across its surface, polymer backings, owing to their more supple nature, tend not to lie flat and may and/or fail to lie with a consistent stress or tautness across their surface. Furthermore, unlike paper whose porosity allows for the escape of air trapped between the prepreg material and the paper, polymer films provide no path of escape and, consequently, air pockets form that are then wound into the subsequently formed master rolls.
Each of the foregoing factors affects the integrity of the interface between the prepreg material and the polymer backing. For example, folds in or bunching up of the polymer backing and/or the presence of air pockets trapped between the prepreg material and the backing create areas where there is no direct contact or adherence between the prepreg and the backing. In master rolls, this is not an issue, but in forming slit tape it would be devastating. Specifically, were one to try to slit these materials without removing the backing, the lack of adherence between the backing and prepreg material allows for relative motion or movement of one to the other during the slitting process which adversely affects the final dimensions of the slit material. Depending upon the extent of the bunching of the polymer backing and/or the size of the area lacking contact, this could also lead to a jam in the slitter, necessitating one to shut down the process to rectify the situation. A jam also raises the risk for compromising the slit tape structurally and dimensionally, including those slit tape tows that would otherwise have been in specification. Furthermore, assuming that the problem area passed through the slitter without problem, without a bond between the prepreg material and the backing, there is the likelihood, if not certainty, of misalignment between the two as the slit tape is being wound. Consequently, the opportunity exists for prepreg on prepreg in such windings whereby the successive layers in contact with each other bond or meld together making the roll unsuitable for subsequent manufacturing applications. Hence, it is critical to remove the polymer backing to slitting as well.
Equally critical to the typical prepreg slitting operation is the need to insert a new liner, a polymer liner or, more commonly, a polyliner, as the slit tape is being wound so as to prevent bonding or melding of one layer of slit tape to another. This interlining or interleafing process, as it is commonly referred, requires great precision both in terms of alignment and tension of the liner and slit tape as one must directly align the narrow strip of slit tape with the liner as well as ensure good mating between the two so that the two do not move relative to one another in the final winding step. In part, this is addressed by employing a liner material that is somewhat wider than the slit tape. However, even that has its limitations as one can only economically and functionally use a liner whose width is marginally wider than the slit tape, perhaps 10 to 20 percent, in the case of narrow tapes, somewhat less in the case of wide tapes. Though the additional width is quite small, over the miles and miles of slit tape produced the added materials costs begin to add up.
Additionally, the use of wider liner material affects the consistency or evenness of the winding, particularly in helical windings, as ridges and valleys form reflecting those areas of the winding where slit tape is and is not present. Similarly, in spool windings, it is difficult to directly align the slit tape over itself as the outer edges of the liner orient the winding as it occurs. Lastly, since most specifications for slit tape are weight and/or size limited, the excess liner affects the amount of prepreg slit tape that can be wound on any given spool or spindle.
The second key factor in addressing alignment concerns and the integrity of the contact between the slit tape and liner is the liner feed and alignment system itself. Owing to the speed of these systems as well as the narrow widths of slit tape being produced, liner feed and alignment systems require a large degree of sophistication in order to properly align the liner with the slit tape and to do so under a constant tension to ensure good liner to slit tape contact across the whole of the width of the slit tape. Yet, suitable liner feed and alignment systems entail significant capital expense: an expense that is only multiplied as each winding station must have its own liner feed and alignment capability. Thus, if one has 64 winding heads, one needs 64 liner feed and associated alignment systems.
Thus, despite all the advances that have been made, there still remains a need in the industry for a simpler, less capital intensive and less costly process and apparatus for producing of slit tapes.
More importantly, there still remains a need in the industry for such a process and apparatus that can be implemented without a significant capital investment and without compromising or sacrificing production rate and quality.